Can anyone explain what exactly is going on physiologically that makes our muscles tighten up? What exactly is happening when we feel that a muscle is tight? What's happening inside the muscle tissue to make it tighten up, and why does that happen?

Most of us get tight muscles from running, and don't get tight muscles from swimming. I know that the forces generated from running are fairly large, and that imperfect form or biomechanics can create large stresses on muscles and connecting tissue. What I don't understand is why those stresses cause the muscles to tighten, and what exactly is happening when a muscle is tight.

On a related note, why does applying pressure (aka massage) help relieve muscle tightness?

Thanks,
Lee